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**Identifying Quadratic Functions**

9-1 Identifying Quadratic Functions Warm Up Lesson Presentation Lesson Quiz Holt Algebra 1

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Warm Up 1. Evaluate x2 + 5x for x = 4 and x = –3. 36; –6 2. Generate ordered pairs for the function y = x2 + 2 with the given domain. D: {–2, –1, 0, 1, 2} x –2 –1 1 2 y 6 3

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Objectives Identify quadratic functions and determine whether they have a minimum or maximum. Graph a quadratic function and give its domain and range.

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Vocabulary quadratic function parabola vertex minimum maximum

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**The function y = x2 is shown in the graph**

The function y = x2 is shown in the graph. Notice that the graph is not linear. This function is a quadratic function. A quadratic function is any function that can be written in the standard form y = ax2 + bx + c, where a, b, and c are real numbers and a ≠ 0. The function y = x2 can be written as y = 1x2 + 0x + 0, where a = 1, b = 0, and c = 0.

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In Lesson 5-1, you identified linear functions by finding that a constant change in x corresponded to a constant change in y. The differences between y-values for a constant change in x-values are called first differences.

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Notice that the quadratic function y = x2 doe not have constant first differences. It has constant second differences. This is true for all quadratic functions.

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**Example 1A: Identifying Quadratic Functions**

Tell whether the function is quadratic. Explain. Since you are given a table of ordered pairs with a constant change in x-values, see if the second differences are constant. x y –2 –9 +1 +7 +1 –6 +0 +6 –1 –2 –1 1 Find the first differences, then find the second differences. 2 7 The function is not quadratic. The second differences are not constant.

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**Be sure there is a constant change in x-values before you try to find first or second differences.**

Caution!

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**Example 1B: Identifying Quadratic Functions**

Tell whether the function is quadratic. Explain. Since you are given an equation, use y = ax2 + bx + c. y = 7x + 3 This is not a quadratic function because the value of a is 0.

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**Example 1C: Identifying Quadratic Functions**

Tell whether the function is quadratic. Explain. y – 10x2 = 9 Try to write the function in the form y = ax2 + bx + c by solving for y. Add 10x2 to both sides. + 10x x2 y – 10x2 = 9 y = 10x2 + 9 This is a quadratic function because it can be written in the form y = ax2 + bx + c where a = 10, b = 0, and c =9.

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**Only a cannot equal 0. It is okay for the values of b and c to be 0.**

Helpful Hint

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**Tell whether the function is quadratic. Explain. **

Check It Out! Example 1a Tell whether the function is quadratic. Explain. Since you are given a table of ordered pairs with a constant change in x-values, see if the second differences are constant. x y –2 4 +1 –3 –1 +1 +3 +2 –1 1 1 1 Find the first differences, then find the second differences. 2 4 The function is quadratic. The second differences are quadratic.

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Check It Out! Example 1b Tell whether the function is quadratic. Explain. y + x = 2x2 Try to write the function in the form y = ax2 + bx + c by solving for y. Subtract x from both sides. – x – x y + x = 2x2 y = 2x2 – x This is a quadratic function because it can be written in the form y = ax2 + bx + c where a = 2, b = –1, and c = 0.

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**The graph of a quadratic function is a curve called a parabola**

The graph of a quadratic function is a curve called a parabola. To graph a quadratic function, generate enough ordered pairs to see the shape of the parabola. Then connect the points with a smooth curve.

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**Example 2A: Graphing Quadratic Functions by Using a Table of Values**

Use a table of values to graph the quadratic function. Make a table of values. Choose values of x and use them to find values of y. x y 4 3 1 –2 –1 1 2 Graph the points. Then connect the points with a smooth curve.

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**Example 2B: Graphing Quadratic Functions by Using a Table of Values**

Use a table of values to graph the quadratic function. y = –4x2 x –2 –1 1 2 y –4 –16 Make a table of values. Choose values of x and use them to find values of y. Graph the points. Then connect the points with a smooth curve.

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Check It Out! Example 2a Use a table of values to graph each quadratic function. y = x2 + 2 Make a table of values. Choose values of x and use them to find values of y. x –2 –1 1 2 y 2 3 6 Graph the points. Then connect the points with a smooth curve.

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Check It Out! Example 2b Use a table of values to graph the quadratic function. y = –3x2 + 1 Make a table of values. Choose values of x and use them to find values of y. x –2 –1 1 2 y 1 –2 –11 Graph the points. Then connect the points with a smooth curve.

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As shown in the graphs in Examples 2A and 2B, some parabolas open upward and some open downward. Notice that the only difference between the two equations is the value of a. When a quadratic function is written in the form y = ax2 + bx + c, the value of a determines the direction a parabola opens. A parabola opens upward when a > 0. A parabola opens downward when a < 0.

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**Example 3A: Identifying the Direction of a Parabola**

Tell whether the graph of the quadratic function opens upward or downward. Explain. Write the function in the form y = ax2 + bx + c by solving for y. Add to both sides. Identify the value of a. Since a > 0, the parabola opens upward.

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**Example 3B: Identifying the Direction of a Parabola**

Tell whether the graph of the quadratic function opens upward or downward. Explain. y = 5x – 3x2 Write the function in the form y = ax2 + bx + c. y = –3x2 + 5x a = –3 Identify the value of a. Since a < 0, the parabola opens downward.

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Check It Out! Example 3a Tell whether the graph of the quadratic function opens upward or downward. Explain. f(x) = –4x2 – x + 1 f(x) = –4x2 – x + 1 Identify the value of a. a = –4 Since a < 0 the parabola opens downward.

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Check It Out! Example 3b Tell whether the graph of the quadratic function opens upward or downward. Explain. y – 5x2 = 2 x – 6 Write the function in the form y = ax2 + bx + c by solving for y. Add 5x2 to both sides. y – 5x2 = 2 x – 6 + 5x x2 y = 5x2 + 2x – 6 Identify the value of a. a = 5 Since a > 0 the parabola opens upward.

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**The highest or lowest point on a parabola is the vertex**

The highest or lowest point on a parabola is the vertex. If a parabola opens upward, the vertex is the lowest point. If a parabola opens downward, the vertex is the highest point.

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**Example 4: Identifying the Vertex and the Minimum or Maximum**

Identify the vertex of each parabola. Then give the minimum or maximum value of the function. A. B. The vertex is (–3, 2), and the minimum is 2. The vertex is (2, 5), and the maximum is 5.

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Check It Out! Example 4 Identify the vertex of each parabola. Then give the minimum or maximum value of the function. a. b. The vertex is (–2, 5) and the maximum is 5. The vertex is (3, –1), and the minimum is –1.

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Unless a specific domain is given, you may assume that the domain of a quadratic function is all real numbers. You can find the range of a quadratic function by looking at its graph. For the graph of y = x2 – 4x + 5, the range begins at the minimum value of the function, where y = 1. All the y-values of the function are greater than or equal to 1. So the range is y 1.

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**Example 5: Finding Domain and Range**

Find the domain and range. Step 1 The graph opens downward, so identify the maximum. The vertex is (–5, –3), so the maximum is –3. Step 2 Find the domain and range. D: all real numbers R: y ≤ –3

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Check It Out! Example 5a Find the domain and range. Step 1 The graph opens upward, so identify the minimum. The vertex is (–2, –4), so the minimum is –4. Step 2 Find the domain and range. D: all real numbers R: y ≥ –4

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Check It Out! Example 5b Find the domain and range. Step 1 The graph opens downward, so identify the maximum. The vertex is (2, 3), so the maximum is 3. Step 2 Find the domain and range. D: all real numbers R: y ≤ 3

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Lesson Quiz: Part I 1. Is y = –x – 1 quadratic? Explain. 2. Graph y = 1.5x2. No; there is no x2-term, so a = 0.

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Lesson Quiz: Part II Use the graph for Problems 3-5. 3. Identify the vertex. 4. Does the function have a minimum or maximum? What is it? 5. Find the domain and range. (5, –4) max; –4 D: all real numbers; R: y ≤ –4

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